From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 42453C61DA4 for ; Fri, 17 Feb 2023 02:15:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230320AbjBQCPO (ORCPT ); Thu, 16 Feb 2023 21:15:14 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42262 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229580AbjBQCPM (ORCPT ); Thu, 16 Feb 2023 21:15:12 -0500 Received: from mail-yw1-x1136.google.com (mail-yw1-x1136.google.com [IPv6:2607:f8b0:4864:20::1136]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7FE174FC92 for ; Thu, 16 Feb 2023 18:15:11 -0800 (PST) Received: by mail-yw1-x1136.google.com with SMTP id 00721157ae682-5338dd2813dso43303317b3.5 for ; Thu, 16 Feb 2023 18:15:11 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=gIW9E+wxkNTINaEXAot3lldoZz6sgaV4NvNtgjJaTFE=; b=HZ1MbnjL/We5C6AX8xTKc/NqTWr1JbNL6JlhMYjY4MPeRhcx4XZI09qb39bGwvRHpn 8cDtil8U0r6ullafxJ+QwLrNcyZB/iYRlwQxWBWKusbbvKu5LjqUSwq9Ra4ShiIiGrEv AxeeaI71BN3x7atdH3dnzKmeE+EolPr5w0YLihfjx8Lqhfdtg4nJ/eIYXfAJrzuibZs7 cByt+UC1cn5tWYrIXSmdfIJG8w44wiSxOwaDq0TQPCM1gjyd36andjC6MWF3zwokrgru 63sqYZasEEiF/RtcTOEDIThAitqANjlx8nQcaAyiwezVllR+cWq9GK3mwpXFnbRC0O/W bLkw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=gIW9E+wxkNTINaEXAot3lldoZz6sgaV4NvNtgjJaTFE=; b=FFBwqV2VABDggM4ZVaNwQ1vnt9AfJC/LQLBOCnmSCaCyr6DtZGWifXpq5UbPlf5yxm m8mOzbT8zgK+aAm9p91m3TgDR1wmfzPcni08sW0Yh60OHAjBI/xz1soOc7cWUY1KIoQm dB5ih93Omc+lcjqJtaTAV4drMZ0mr3D/R9dMoK5VcNpyrOd6HO7T/eOR75fdSmRN23xj xQ29I0PPa6EFl5KiORo9EZPIwQTd9HpyfYCHvkk+RHlEQ2ZTxqSmsFNWBXz9OYlIKFFj mJGZS2Z3EgQUND2ikBt1FOrSgbPoBS0PSPTZRYI1fcK0xIy7qSmlEQoLPAhC+AS/sO09 cJ5w== X-Gm-Message-State: AO0yUKUm4Lur7X3qMaMVhfU1GQUpYKUwRzAELH1pZ9dVeqJqO01vvGwi 4MtF1vwNi3yWqCUPQzskSoIdjrUjkkmqUc45eINHWA== X-Google-Smtp-Source: AK7set9iDocdfaltT1vBZM1FfqGEUR3CgAtWXk18fjpJx7Nf/An2UuUbR7/i7tvhGQPyuOkqzQZqqs2JkP/rFvIUCdY= X-Received: by 2002:a81:4ed4:0:b0:506:6364:fda3 with SMTP id c203-20020a814ed4000000b005066364fda3mr971621ywb.72.1676600110454; Thu, 16 Feb 2023 18:15:10 -0800 (PST) MIME-Version: 1.0 References: <20230216051750.3125598-1-surenb@google.com> <20230216051750.3125598-27-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Thu, 16 Feb 2023 18:14:59 -0800 Message-ID: Subject: Re: [PATCH v3 26/35] mm: fall back to mmap_lock if vma->anon_vma is not yet set To: Matthew Wilcox Cc: akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, chriscli@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, rppt@kernel.org, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, leewalsh@google.com, posk@google.com, michalechner92@googlemail.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 16, 2023 at 11:43 AM Suren Baghdasaryan wrote: > > On Thu, Feb 16, 2023 at 7:44 AM Matthew Wilcox wrote: > > > > On Wed, Feb 15, 2023 at 09:17:41PM -0800, Suren Baghdasaryan wrote: > > > When vma->anon_vma is not set, page fault handler will set it by either > > > reusing anon_vma of an adjacent VMA if VMAs are compatible or by > > > allocating a new one. find_mergeable_anon_vma() walks VMA tree to find > > > a compatible adjacent VMA and that requires not only the faulting VMA > > > to be stable but also the tree structure and other VMAs inside that tree. > > > Therefore locking just the faulting VMA is not enough for this search. > > > Fall back to taking mmap_lock when vma->anon_vma is not set. This > > > situation happens only on the first page fault and should not affect > > > overall performance. > > > > I think I asked this before, but don't remember getting an aswer. > > Why do we defer setting anon_vma to the first fault? Why don't we > > set it up at mmap time? > > Yeah, I remember that conversation Matthew and I could not find the > definitive answer at the time. I'll look into that again or maybe > someone can answer it here. After looking into it again I'm still under the impression that vma->anon_vma is populated lazily (during the first page fault rather than at mmap time) to avoid doing extra work for areas which are never faulted. Though I might be missing some important detail here. > > In the end rather than changing that logic I decided to skip > vma->anon_vma==NULL cases because I measured them being less than > 0.01% of all page faults, so ROI from changing that would be quite > low. But I agree that the logic is weird and maybe we can improve > that. I will have to review that again when I'm working on eliminating > all these special cases we skip, like swap/userfaults/etc.